A suitable format for a communication in a wireless network is the ZigBee Specification (053474r06) for low cost, low power wireless radio links, as defined by the ZigBee Alliance. It uses functionality described in IEEE Standard 802.15.4-2003. Communication between nodes takes place over one of a number of “physical channels”. The 2.4 GHz communication band contains 16 separate 802.15.4 channels, so a number of Personal Area Networks (PANs) can operate simultaneously with each on its own channel and so not interfering with each other.
It is possible for more than one PAN to use a single channel, especially if they are a long distance apart (>10-100 m away), or if both have fairly low usage rates (low duty cycle).
A problem with ZigBee is that WiFi devices also operate in the 2.4 GHz band, and WiFi traffic can interfere with ZigBee traffic. WiFi messages can be transmitted at a much higher power level, and a WiFi logical channel is as wide as several ZigBee logical channels.
In order for two such systems to coexist it is beneficial to be able to work out what channels are clear, and get the interfered-with network (e.g. the ZigBee PAN) to operate on these channels.
ZigBee devices have the ability to assess which channels are clear—they carry out an “Energy detection scan”, which returns a list containing the background power level on each available channel. It would therefore be possible for the PAN Coordinator to carry out a scan and simply use the clearest channel, but this is not very good for a large network, as channel conditions vary with location, and at a different point in the network the selected channel may be unusable.
Ideally, the PAN Coordinator (or other commissioning tool) would contact each device in the network, and request the results of an energy scan. Unfortunately this is only possible if reliable communications are operational between all devices, in which case no change is necessary anyway. Currently no good solution exists for getting energy levels from devices for which there is not already a reliable connection.
The network also needs to handle the situation where a device is orphaned (unable to communicate with the network) due to a local communication issue. This might be, for example, due to a WiFi router having been setup nearby. An orphaned device needs to somehow re-establish communication with the network to inform the PAN Coordinator that it is unable to reliably communicate on the current channel.